The end goal of communist theory is a stateless, moneyless society. The fact that no country has been successful in transitioning to such a society doesn't matter here, the end goal remains the same, when speaking about theory.
I personally maintain the major downfall has been putting one person in charge. That's never gone particularly well, even in capitalist democracies. I'd like to see a country try it out with a council at the top, preferably 9 or more members, but always an odd number to prevent ties.
Also they all failed to establish a proper democracy first, and therefore fell into authoritarianism